You're looking at a once-in-a-lifetime scientific discovery. A team of experts announced today …
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The researchers believe the Tissint Martian Meteorite was ejected from Mars some 700,000 years ago, and its composition is uniquely complex. The meteorite has three distinct different types of deposit, linked to different areas of Mars. The researchers think this can be blamed on Martian weathering, and that this hunk of rock was once heavily weathered by water on Mars. In the paper, the researchers conclude:

We propose the following scenario in order to explain the composite nature of Tissint. A picritic basalt was emplaced at or near the surface of Mars. After some period, the rock was weathered by fluids, which had leached elements from the Martian regolith. Subsequently, these fluids deposited mineral phases within fissures and cracks.

When the meteorite hit Earth, shock-induced melting spread through the weathering fractures, melting them into black glass. Even in this changed form, the unique chemical signatures remained, allowing the scientists to reconstruct the path of Tissint.

Every other example of a Martian meteorite has suffered weathering on our planet, too, which makes the Tissint sample one of the most pristine ever seen.